Circuit protection devices in HVAC control applications are well known. HVAC control circuits contain elements which may be harmed by overcurrents. Conventional fuses were developed to protect such elements by creating an open circuit in response to an overcurrent, but suffer from the drawback of being single-use devices. Once an open circuit is created in the fuse element, the fuse must be replaced. This deficiency was addressed with the invention of conventional circuit breakers. However, circuit breakers typically must be manually reset after "tripping"; where "tripping" is defined as creating an open circuit or limiting the current in the circuit to a minimal, non-damaging value. Both fuses and circuit breakers are oftentimes located in places which are difficult or dangerous for service personnel to access. Additionally, circuit breakers are prone to failure modes typically associated with mechanical devices. Further, circuit breakers are typically bulky items compared to fuses.
PTC materials, specifically polymer-based PTC materials, exhibit characteristics which offer advantages over both conventional fuses and circuit breakers. PTC materials behave such that a steady state trip current, I.sub.trip, through PTC material which will cause the material to "trip" can be chosen to be less than the maximum current carrying capacity, I.sub.max, of an HVAC control circuit to be protected and greater than a combined load current, I.sub.load, drawn by all loads in the HVAC control circuit; where I.sub.max is greater than I.sub.load. Similar characteristics can generally be found in both fuses and circuit breakers.
Upon encountering a current greater than I.sub.trip, the resistance of PTC material increases to limit current to a steady-state value that can be chosen to be substantially lower than I.sub.trip, I.sub.load, or I.sub.max. Unlike fuses or circuit breakers, removal of conditions which caused the PTC material to "trip" causes the material to return to a state allowing currents up to I.sub.trip to be conducted. Unlike fuse material, PTC material may be reused. Unlike circuit breakers, a PTC material-based device does not require service personnel or the user to access the device to reset it. PTC materials sufficient to provide overcurrent protection in HVAC control circuits are typically much less bulky than comparable circuit breakers. Current state of the art includes PTC materials which have a voltage rating commensurate with that required by HVAC control circuits.
Therefore, in view of the need for multi-use, non-bulky, HVAC control circuit overcurrent protection which can be reset without the user having access to the device and the potential benefits to be enjoyed in addressing the drawbacks of conventional circuit protection devices used in HVAC control circuits, the present invention has been designed and developed.